1. Field of the Invention
This invention relates to an image display device for displaying an image such as a picture image, a projected image or the like, and more particularly to an image display device which is adapted to carry out addressing of a luminous point by a grid or deflecting electrode group using an electron beam accelerated under a high voltage, resulting in a display of a picture image, a projected image or the like. The present invention is suitable for use for a wall-mounting type television receiver, a high definition television, an OA (office automation) display, an FA (factory automation) display, a CAD (computer aided design) display and the like.
2. Background of the Invention
A display device of such type for a picture image or a projected image which has been conventionally known in the art generally includes a CRT (cathode ray tube) type display device which utilizes a so-called Braun tube.
The CRT type display device adapted to display a picture image or a projected image is so constructed that at least one electron beam emitted from an electron gun is scanned on a display plane of a screen having phosphors deposited thereon and accelerated and controlled electrons are impinged on a phosphor coated surface of the screen at a high velocity, to thereby carry out a luminous display. Such construction of the conventional CRT type display device permits the device to utilize a phosphor excited by a high velocity electron, resulting in readily accomplishing a color display of high luminance and high definition.
However, in the CRT type display device, not only the electron gun is provided with control means and arranged behind the screen, but scanning of the electron beam emitted from the electron gun in a manner to extend to an end of the screen requires the device to have a considerable depth, to thereby render thinning or lightening of the device highly difficult.
A graphic display device is proposed for the purpose of providing a thinly-made or thin display device substituted for the CRT type display device, to which a principle of a fluorescent display device is applied.
Such a fluorescent display device for graphic display utilizes a ZnO:Zn phosphor which is adapted to exhibit sufficient luminance when a low velocity electron impinges thereon, resulting in a monochromatic display. It exhibits sufficient utility when it is formed into a small size. However, it is driven at a low voltage and utilizes a flood electron beam or a diffused electron beam, so that the prior art substantially fails to provide a fluorescent display device of a relatively large size and a thin shape for a color graphic display.
More particularly, in the conventional fluorescent display device, an increase in the number of picture cells on a picture plane caused by large-sizing of the screen necessarily reduces its duty ratio, resulting in decreasing average luminance on the whole picture plane even when luminance of each picture cell is instantaneously increased. Accordingly, when it is desired to obtain required luminance on the picture plane, it is required to carry out emission of higher luminance from each picture cell. Unfortunately, a color phosphor excited by a low velocity electron is low in luminous efficiency, accordingly, driving of the phosphor at such a low voltage as described above fails to cause it to exhibit high luminance. Also, the phosphor fails to have a long life because it is rapidly deteriorated, as well as causes cathode pollution by decomposition products due to deterioration of the phosphor.
In view of the above, a flat-type color image display device is recently proposed which is formed into a thin shape as compared with the CRT type display device, driven at a high voltage and carries out addressing of a luminous section using an electrode group including a grid, a deflection electrode and the like.
FIG. 11 is an exploded perspective view schematically showing a basic structure of such a flat-type color image display device.
The color image display device generally includes an electrode group including a phosphor screen section 1 having a phosphor and an accelerating electrode deposited on an inner surface thereof and acting as a display plane, a plurality of filamentary cathodes 2 spaced from the phosphor screen section 1 and stretchedly arranged in a manner to be vertically spaced from each other and each horizontally extend along the phosphor screen section 1, a back electrode 3 arranged for directing electrons emitted from each of the filamentary cathodes 2 toward the phosphor screen section 1, a vertical focusing electrode 4 for focusing the emitted electrons in the vertical direction to form them into each electron beam, a vertical deflecting electrode 5 for deflecting the electron beams in the vertical direction, a horizontal selecting electrode 6 for selecting a horizontal direction of a display plane, a horizontal deflecting electrode 7 for deflecting the electron beams in the horizontal direction, and the like. The electrode group is received in a flat-type box-like envelope of which an interior is evacuated to and kept at a high vacuum.
The conventional color image display device constructed as described above is so operated that each electron beam emitted from each of the filamentary cathodes 2 is selectively impinged on each of picture cells defined on the inner surface of the phosphor screen section 1 at a high velocity while being controlledly deflected in the vertical and horizontal directions by the electrodes 4 to 7, resulting in a desired luminous display.
In the conventional color image display device, the back electrode 3 and vertical focusing electrode 4 each are formed by a common single plate and a predetermined voltage is applied to the back electrode 3 and vertical focusing electrode 4, and further a pulse voltage is applied to a plurality of the filamentary cathodes 2 from one end of each of the cathodes to the other end thereof in order, so that the electron beams are fed in order depending on a relationship in potential between the back electrode 3 and the vertical focusing electrode 4, resulting in selecting a vertical position on the display plane.
However, techniques for manufacturing the conventional color image display device certainly fail in proper or uniform aligning of the cathodes 2 with the back electrode 3 and vertical focusing electrode 4, so that each of the cathodes 2 are not necessarily uniformly or regularly positioned with respect to the electrodes. Also, there occurs a dispersion in electron emitting capacity among the cathodes 3. This causes the amount of electrons fed from the respective cathodes 2 through the vertical focusing electrode 4 toward the phosphor screen section 1 to be varied depending on the cathode.
Unfortunately, it is highly difficult to adjust the amount of electrons contained in each electron beam taken out through the vertical focusing electrode 4 because each of the back electrode 3 and vertical focusing electrode 4 is formed by a common single plate as described above, resulting in unevenness in luminance tending to occur on the display plane. Further, in the conventional color image display device, as described above, electrons emitted from each of the cathodes 2 is shaped into an electron beam by the vertical focusing electrode 4 and horizontal selecting electrode 6, which is then selectively impinged on the picture cells on the phosphor screen section 1 while being accelerated by the section 1 after it is deflected by the vertical deflecting electrode 5 and horizontal deflecting electrode 7, resulting in a desired luminous display. In this instance, in order to facilitate selective control of each electron beam and controlled deflection of it, it is desired to carry out the above-described operation at a low voltage.
However, the above-described construction of the conventional display device causes the horizontal deflecting electrode 7 to be affected by a region of the phosphor screen section 1 to which a high voltage is applied because the electrode 7 is exposed directly to the phosphor screen section 1 also serving as an accelerating electrode, to thereby render the horizontal deflection at such a low voltage as described above difficult. This leads to a decrease in deflecting sensitivity of the horizontal deflecting electrode 7, as well as tends to cause electrical discharge between the horizontal deflecting electrode 7 and the phosphor screen section 1. Such discharge adversely affects a deflecting circuit connected to the horizontal deflecting electrode 7 and the like to often lead to damage of the circuit.
As an approach to such disadvantages, it would be considered that a protective electrode is interposedly arranged between the horizontal deflecting electrode 7 and the phosphor screen section 1. However, in this instance, it is highly required to effectively prevent the protective electrode from affecting control for selection and deflection at the address section, accordingly, arrangement of the protective electrode is not desirable.